S.A. Klein

Estimation of the diffuse radiation fraction for hourly, daily and monthly-average global radiation

Hourly pyrheliometer and pyranometer data from four U.S. locations are used to establish a relationship between the hourly diffuse fraction and the hourly clearness index kT. This relationship is compared to the relationship established by Orgill and Hollands and to a set of data from Highett, Australia, and agreement is within a few percent in both cases. The transient simulation program TRNSYS is used to calculate the annual performance of solar energy systems using several correlations. For the systems investigated, the effect of simulating the random distribution of the hourly diffuse fraction is negligible. A seasonally dependent daily diffuse correlation is developed from the data, and this daily relationship is used to derive a correlation for the monthly-average diffuse fraction.

Calculation of monthly average insolation on tilted surfaces

Abstract Several simplified design procedures for solar energy systems require monthly average meteorological data. Monthly average daily totals of the solar radiation incident on a horizontal surface are available. However, radiation data on tilted surfaces, required by the design procedures, are generally not available. A simple method of estimating the average daily radiation for each calendar month on surfaces facing directly towards the equator has been presented by Liu and Jordan [1]. This method is verified with experimental measurements and extended to allow calculation of monthly average radiation on surfaces of a wide range of orientations.

A design procedure for solar heating systems

This paper is concerned with the design of solar space and water heating systems for residences. A simulation model capable of estimating the long-term thermal performance of solar heating systems is described. The amount of meteorological data required by the simulation in order to estimate long-term performance is investigated. The information gained from many simulations is used to develop a general design procedure for solar heating systems. The result is a simple graphical method requiring monthly average meteorological data which architects and heating engineers can use to design economical solar heating systems. A method of estimating the monthly average radiation on tilted surfaces is given in the Appendix.

Calculation of flat-plate collector utilizability

Abstract The “utilizability” or φ-curve method developed by Whillier [1] and later generalized by Liu and Jordan[2, 3], can be a very useful design tool for some types of solar energy systems which utilize conventional flat-plate solar collectors. The generalized φ-curve method in its original form, however, has several drawbacks. The calculation effort required to assess long-term collector performance is significant. The calculations can not be completely automated on a computer or hand calculator because many graphs are needed for which analytical representations are not available. In this paper, the φ-curve method is reviewed and situations for which the method is useful are described. Then, an easier method, using daily φ -charts, rather than the original hourly φ-curves, is presented. The φ -charts greatly reduce the calculations required to determine flat-plate collector utilizability. φ -charts can be easily implemented on a programmable hand calculator. A comparison of the original hourly and the new daily calculation methods is presented. An example demonstrating the use and utility of the φ -charts is also included.

Performance study of one-dimensional models for stratified thermal storage tanks

Two basic approaches are used to model the temperature distribution in thermal storage tanks for solar domestic hot water (SDHW) systems. In the multinode approach, the tank is divided into N nodes, with an energy balance written for each node. This approach results in a set of N differential equations that can be solved for the temperatures of the nodes as a function of time. In the plug flow approach, segments of liquid of different temperatures and sizes are assumed to move through the tank in a plug flow manner. The sizes of the fluid elements are determined mainly by the simulation time step and the flow rates. Whenever the incoming fluid from the heat source is colder than the fluid at the top of the tank, “plume entrainment” occurs. A model describing plume entrainment has been incorporated into both the multinode and the plug flow models in the TRNSYS program[1]. A performance study of the TRNSYS tank models has been carried out with experimental data from two different sources. Three performance numbers have been defined for quantifying the accuracy of the models compared with experimental data. Recommendations are given as to which tank model should be used under which conditions.

A method for estimating the long-term performance of direct-coupled PV pumping systems

Abstract A method is developed to predict the long-term performance of direct-coupled PV pumping systems. The method uses only information available from the PV module and pump-motor manufacturers. Weather data are “generated” from monthly averages of horizontal radiation and ambient temperatures using well-known weather data statistics. The method predicts monthly pumped water to within 6% of TRNSYS predictions based on hourly weather data. The use of a single monthly-average day is shown to underpredict monthly pumped water at low monthly average radiation levels and overpredict monthly pumped water at intermediate radiation levels. Only at high radiation levels does the use of a single monthly-average day provide a reasonable estimation of monthly pumped water.

Transient Considerations of Flat-Plate Solar Collectors

A mathematical study showed that a quasi-steady-state model which assumes zero capacitance adequately represents the performance of a flat-plate solar collector if hourly meteorological data are the best available. The use of more complex models which account for capacitance is justified only if better data can be obtained.

A methodology for the synthesis of hourly weather data

Abstract The ability to generate weather data from limited inputs and location independent correlations would allow simulations of environmentally driven systems to be run at locations for which detailed weather records do not exist. A further improvement would be to generate just one year of data which would yield the same simulation results as those of long-term data, thereby minimizing the computation needed to assess long-term system performance. In this paper, a methodology to calculate such a year of data is described. It is a combinationand modification of several previously published generation methods. Statistics obtained from long-term (22 years), Typical Meteorological Year, and synthetically generated weather data are compared at three locations (Albuquerque, NM, Madison, WI, and New York, NY).

An effectiveness model of liquid-desiccant system heat/mass exchangers

Abstract This paper describes a computationally efficient model for packed-bed, liquid-desiccant heat and mass exchangers. The model is derived from an effectiveness model of a cooling tower. The paper includes the development and derivation of the effectiveness model, comparisons with experimental data and with a finite-difference model.

Absorption power cycles

We present a thermodynamic analysis of the Maloney and Robertson and the Kalina absorption power cycles. The maximum power for specified external conditions is identified and used as a reference to evaluate the performance of these two absorption power cycles. The evaluation focuses on power cycles operating with low temperature heat sources such as geothermal heat, solar energy or waste heat.